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is this voltage drop or bad alternator

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1Gina2G

10+ Year Contributor
810
2
May 6, 2011
Beaufort, South_Carolina
so after a good drive, with the engine warmed up, when the headlights are switched off, the idle changes by sounding a little bit faster then it did, even though the idle was never sounding bad at all.

the battery reads 14.3-14.4V upon initial start up with everything off, goes to 14-14.1V with everything on. strange enough after todays drive i turned everything off, checked voltage and it was at 13.7V, but basically no charging time between shutting off (electrical) and checking voltage, is this why?

battery is brand new, just a few days old, and is holding a charge just fine, no power drains in the car (fixed them)

also all fuses are good

thanks everyone :thumb:
 
The alternator charges around 14v but the battery will not maintain much more than 12.6 after the engine is off. 6 cells in an automotive battery for a voltage of 2.1 per cell.
 
Not sure if you misread anything but when I say turn off I just mean electrical, I'm aware of what the battery voltages drops too when off as I mentioned it also holds a charge.

Honestly I feel like rewiring my alternator with bigger wires (factory wires currently) at the same time, the alternator is used and came with the car, not sure how old it is but it appears to be working and charging the battery. :hmm:
 
Sounds normal. The alternator has a sensing wire to the voltage regulator that tells the alternator how much recovery voltage (based on load) is needed. After a good drive the battery should be near full charge and with all accessories off the load demand is minimal, therefore, the battery requires less overall charging voltage. This is simplified but should answer you well enough for your purposes.
 
It's totally normal, the battery is charged, and you are turning off all the electrical accessories, the voltage has to drop... the regulator and the ECU are working fine...
 
I know how I test my alt is to disconnect the battery while the car is running and if it turns off it means the alt is not working IDK if this helps but it always has worked for me.
 
As by the quote from STEVE:

"Some people think that it is ok to test your alternator by starting the car and then pulling the battery cable from the positive terminal, saying that if the car remains running that the alternator is good.

Like he said, this is a bad idea because the alternator can load dump and spike the whole electrical system with >100v and even if it doesn't, the battery filters the alternator output and the AC ripple on the output will again flow through the whole electrical system. Both of these can damage the ECU and other electronics in the car."

Worst trick in the world to do to any vehicle.

-DSM
 
I know how I test my alt is to disconnect the battery while the car is running and if it turns off it means the alt is not working IDK if this helps but it always has worked for me.

LOL I know you can not do this with any vehicle running an ECU. Old school trick thats now abandonded practically. Glad to hear my issue sounds normal, first DSM so I wouldn't know any better. thanks again everyone! :thumb:
 
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